CS 35L Fall 19 Section 3 Notes Week 3 Monday Zhaowei Tan Python Basics: 1. Installation and package management: sudo pip install module_name sudo pip3 install module_name Note that you might not be able to do that if you do not have root permission (i.e., unable to ‘sudo’) 2. Using interactive shell or script execution: python script.py python3 script.py 3. Modules are important parts of Python, probably one of the major reasons why people love Python. Use pip to install a new module in your computer. To import a module into the script: import module import module as module_alias 4. Variable types: You don’t have to declare the variable type when you assign it – instead, Python would automatically know what type the variable is. E.g. the following all work: a = “123” a = 3 a = 4.0 Note that: 1) To transfer from one type to another, use something explicitly like a = 4.0 a = int(a) 2) In calculation, the Python would not do the auto type transfer for you a = 4 / 3 # equals 1 in python 2 a = 4 / 3 # equals 1.333 in python 3 3) To check the type, use type(var) 5. Use print (…) to print things to standard output. Print can recognize any type of the variable, int, float, list, etc. Python 3 print special: sep and end print(‘G’,’F’,’G’, sep=”) print(“Welcome to” , end = ‘ ‘) 6. String: a. Access a char: s[index]; index can be negative, s[start:end] for slicing b. Split the string: s.split(‘delimiter’) c. Find substring within a string: s.find(“substring”, start_position). Returns index, or negative if not found. Other method: replace, upper, … d. Concatenate: c = a + b 7. List: To create a list my_list = [‘a’, 4, [1, 3]] The items in Python list are not necessarily be of same length or same type. Use list[a] to fetch an item. Use list[a:b] to fetch a subset. Index a is inclusive, but b is exclusive. Useful list functions: append, extend, index, remove, pop, count… 8. Dictionary To initiate a list/directory my_dict = {“name”: “Zhaowei”, “major”: “CS”, “status”: 0} Access value by key: my_dict[“name”] Useful dictionary functions: keys, values, items… 9. Tuples: Not mutable. Old tradition of keeping heterogeneous stuffs. Can be used as key for dictionary. 10. if condition: do something You can use and, or, not to connect the different statements. Use indentation to wrap the block. To enable else if: if condition1: do something elif condition2: do something else else: do something different Some special conditions: if key in dict (check if key is in the keys of dict); if ele in list 11. Loops: while condition: do something or for i in [list]: do something for i Note: 1) Be careful that Python uses indentation to indicate different levels, instead of using brackets (C) or keywords (Bash). I recommend using spaces instead of tabs as your indentation for compatibility. 2) You could use range(min, max, step) to generate a number list and use for loop to iterate. The list generated includes min, but not max (similar to list[a:b]). We can use a negative step to generate numbers in a reversed order. 3) To terminate a while/for loop, use break. To skip the current pass of loop, use continue. 12. Functions in Python: def func_name(arg1, arg2, …): do something return something Call this function func_name(arg1, arg2, …) You don’t have to specify the type for arguments. 13. Python class class className: define your variables define your functions A special function is called __init__, which will be called after you instantiate a new variable. The functions in the class should have an extra argument, self Inside the function, to call the variable inside the class, use self.varName When you put double underscore (__) before the variable name, the name of the variable changes from __varName to _className__varName outside this class. This is a common practice to declare the variable as a private variable, however, we could still access this variable using the transferred variable name. But never do that! The same for function here. A class example: class myClass: __num = 0 def __init__(self, num): self.__num = 100 def printNum (self): print self.__num def _add_people (self, num): self.__num += num t = myClass(100) t. printNum() t._add_people(10) print t._myClass__num #don’t do this in reality! 14. Exception handling. We could use try/except to capture the error and continue the program. try: f = open(“testfile”, “w”) f.write(“This is a test file.”) except IOError: print “Error: cannot find file or read data” else: print “Written content in the file successfully” f.close() Note: this can only catch runtime error, not syntax error
